The present invention relates to a method for production of elongated ceramic sheets. An object of the invention is to improve the doctor-blade method and thereby to provide an elongated ceramic sheet with few undulations or warps. The present invention also relates to a method for production of elongated superconducting sheets.
Conventionally, ceramic sheets, utilized as substrates in the electronics field, are manufactured by the doctor-blade method. In the method, a ceramic powder, such as Al.sub.2 O.sub.3 or ZrO.sub.2, is first mixed with a medium (example: water). The mixture is then mixed with other necessary compounds, such as binders or plasticizers. As a result, a uniform slurry is obtained. The slurry is then painted at a prescribed thickness on a carrier tape using a doctor-blade. Next, the medium is evaporated by hot air or infrared rays, so that the slurry is hardened to a green sheet (unsintered ceramic sheet). Then, the green sheet obtained is cut to prescribed dimensions, and the cut green sheets are sintered in a sintering furnace so as to form the ceramic sheets.
In the conventional doctor-blade method, several problems exist as described hereinafter.
The length of the ceramic sheet to be obtained is limited by the size of the sintering furnace. In other words, it is impossible to make a sheet longer than the length of the sintering furnace.
The green sheet varies in composition through its thickness due to gravitational-setting effects during hardening. Because of this nonuniformity of composition, the ceramic sheet comes to have undulations or warps caused by differential expansion of the faces of the green sheet during sintering, as shown in FIG. 12. In FIG. 12, numeral 35 denotes a sintered ceramic with warped edges.
Recently, various oxide superconductors having critical temperatures higher than the temperature of liquid nitrogen have been discovered (for example, the Y-Ba-Cu-O group, the Bi-Sr-Ca-Cu-O group, and the Tl-Ba-Ca-Cu-O group).
Conventionally, the oxide superconductors are produced by one of the following methods:
(1) A method wherein the material powder is mixed, shaped into a final form, and then sintered.
(2) A method wherein the mixed material powder obtained by coprecipitation is shaped into a final form and then sintered.
(3) A method wherein a thin superconducting film is formed on a platelike substrate by sputtering, chemical vapor deposition, or other deposition process.
Oxide superconductors produced by the above methods have poor mechanical strength and elasticity. Therefore, if such superconductors are formed into elongated shapes, like sheets or ribbons, the superconducting sheets are too fragile for practical use.